liblog: Add private interfaces for buffer size properties

Add private function __android_logger_get_buffer_size() to read
properties and compose the default buffer size.  This interface
complements the existing android_looger_get_size() which returns
the logd setting which can differ at runtime.  For use in logd
and dumpstate.  Side effect is we also add the private functions
__android_logger_property_get_bool() and
__android_logger_valid_buffer_size() for reuse in logd.

Test: gTest liblog-unit-test, logd-unit-tests and logcat-unit-tests in
      combination with commit 'logd: Use private interfaces for
      buffer size properties'
Bug: 31750617
Change-Id: Id95cb68f775ef6b427c122e10f6f8291d336d184