soong: add support for module specific lexer flags

To support module specific lexer flags, this follows the same strategy
as the yacc flags:
 - add LexProperties to the BaseCompilerProperties
 - propagate those flags to the generator generation (i.e. genLex)
 - add a placeholder for custom flags
 - replace the placeholder with the concatenated flags

This might not support escaping very well, but I figured that this is a
very edge case. Support for escaping etc. could be added later on.

Bug: 159682555
Signed-off-by: Matthias Maennich <maennich@google.com>
Change-Id: I31a3b783bb05213fe1621191031952b41b318103