Bug 1643811 - [ci] Add a SCHEDULES rule to prevent 'android' builds and tests on pushes that exclusively modify files under /browser, r=gbrown,Gijs

This rule states that if a push *only* modifies files under the /browser directory, then
we'll only schedule tasks that:

A) Use the 'skip-unless-schedules' optimization (this includes most builds and tests)
B) Are tagged with at one of the 'linux', 'macosx' or 'windows' components

This should be everything except Android builds and tests.

Differential Revision: https://phabricator.services.mozilla.com/D78906

browser/moz.build

@@ -8,9 +8,6 @@ CONFIGURE_SUBST_FILES += ['installer/Makefile']
 
 SPHINX_TREES['/browser'] = 'docs'
 
-with Files('docs/**'):
-    SCHEDULES.exclusive = ['docs']
-
 EXTRA_COMPONENTS += [
     'l10n-registry.manifest',
 ]
@@ -59,6 +56,10 @@ FINAL_TARGET_FILES.defaults += ['app/permissions']
 
 with Files("**"):
     BUG_COMPONENT = ("Firefox", "General")
+    SCHEDULES.exclusive = ['linux', 'macosx', 'windows']
+
+with Files('docs/**'):
+    SCHEDULES.exclusive = ['docs']
 
 with Files("Makefile.in"):
     BUG_COMPONENT = ("Firefox Build System", "General")