CI: run lint and unit tests; add the first tests #34

The project has no tests, and CI only runs bundleRelease and assembleRelease. Lint currently reports several errors (see #29, #30), and nothing would catch new ones.

Proposal:

• Add JUnit tests for the pure logic: SDP parsing, AudioConfig, G.711 tables, AAC AU-header and ADTS parsing, RTP header length, NalAssembler. fix(audio): AAC/G.711 playback; docs: README in EN/RU/ZH, device report form #6 already verified most of this with an ad-hoc JVM harness, which can become the first tests.
• Run ./gradlew testDebugUnitTest lintDebug in CI on every push and pull request. Start with a lint baseline, so that only new problems fail the build.