Southeast Asian leaders flew into Manila for their first regional summit meeting in a decade as thousands of troops mobilized to ensure their safety in the troubled Philippine capital. The leaders of the six-member Assn. of South East Asian Nations, headed by Philippine President Corazon Aquino, are expected to concentrate on trade and economic issues in the third summit of the organization's 20-year history. Despite the strong security in place for the two-day ASEAN meeting, men in a taxi lobbed a hand grenade outside the Malaysian Embassy in suburban Makati. There were no casualties.