Remove max_level in favour of extracting it from the Filter

[MarijnS95]

Fixes #80

Documentation for log specifies that all implementations must have a way to initialize log::set_max_level() (because it otherwise defaults to Off), in our case via the with_max_level() builder - and by calling init_once(). Over time android_logger received env_filter support for much more fine-grained control over filtering, making android_logger similar to env_logger in its implementation.

With this change however, ambiguity arises when independently configuring log levels through .with_max_level() and env_filter::Filter. The former acts as an early performance save directly in log::log!() macros while the latter applies additional filtering inside android_logger. In short: log::max_level() must be at least as high as what Filter can process, otherwise messages get lost before they reach the Filter.

env_logger solved this ambiguity by hiding direct access to log::set_max_level() as an implementation detail and initializing it to the highest level that could possibly be processed by the Filter. Mimick this inside android_logger by removing the with_max_level() setter. In addition all internal is_enabled() filtering and related tests are removed because we rely on the log macros themselves to filter, and no longer track max_level inside. Keeping in mind that any user can techincally change log::set_max_level() to increase or decrease global verbosity.

The only complexity remains around __android_log_is_loggable_len() on Android 30 and above (if enabled on the crate by the user - not necessarily based on the actual minimum nor target SDK level yet!) which asks the Android system for env_filter-like per-module (tag) level overrides.

TODO

• Make env_filter non-Optional?
  • Forward its Builder-constructors like filter_level() and filter_module(), just like env_logger.
  • Also create various constructors based on env_logger/env_filter's environment variables? These are less common to be set for Android apps.
  • Revisit the correct default. Error makes sense here.
• Revise tests around level filters, which are now no longer implemented on the AndroidLogger level.
• Update README and crate-documentation samples to match the new API (once finalized).
  • Insert a test / build step that includes the README in these tests, either:
    1. #![doc = include_str!("README.md")];
    2. https://crates.io/crates/cargo-readme.

[MarijnS95]

@Nercury I'd love your input on the general direction (and specifically its justification in the description) of this PR removing max_level and the TODOs outlined, before I commit to finishing it (definitely don't need a full code-review just yet - I'll let you know by pulling it out of Draft status) 🙂